Operating System (OS) concepts for a Computer Programmer interview.

Understanding Operating System (OS) concepts is essential for a Computer Programmer interview. Here’s a breakdown of key OS topics you should be familiar with:

1. Processes and Threads

• Process:
  • Definition: A program in execution. It is an active entity that has its own memory space.
  • States:
    • New: Process is being created.
    • Ready: Process is waiting to be assigned to a processor.
    • Running: Process instructions are being executed.
    • Waiting: Process is waiting for some event (e.g., I/O).
    • Terminated: Process has finished execution.
  • Process Control Block (PCB): A data structure in the OS that contains information about the process (e.g., process state, program counter, CPU registers, memory limits).
• Thread:
  • Definition: The smallest unit of a process that can be scheduled for execution. Threads within the same process share the same memory space.
  • Multithreading: Multiple threads within a single process, allowing for parallel execution.
  • Use Case: Increases application responsiveness and resource utilization.
• Context Switching:
  • Definition: The process of storing and restoring the state (context) of a process or thread, so that execution can be resumed from the same point at a later time.
  • Overhead: Context switching has a performance cost due to the saving and loading of registers, memory maps, and other data.

2. Memory Management

• Virtual Memory:
  • Definition: A memory management technique that gives an application the impression it has contiguous working memory, while in reality, it may be fragmented and even partially stored on disk.
  • Paging: Memory is divided into fixed-sized blocks called pages. The OS uses a page table to map logical addresses to physical addresses.
  • Segmentation: Memory is divided into segments of varying sizes, based on logical divisions like code, data, and stack segments.
• Page Replacement Algorithms:
  • Least Recently Used (LRU): Replaces the page that has not been used for the longest time.
  • First-In-First-Out (FIFO): Replaces the oldest page in memory.
  • Optimal: Replaces the page that will not be used for the longest time in the future (theoretical).
• Swapping: The process of moving processes between main memory and disk when the main memory is full. Swapping allows for more processes to be in memory, but can cause performance degradation if overused (thrashing).

3. Concurrency

• Concurrency:
  • Definition: The ability of the system to handle multiple operations or tasks simultaneously.
  • Issues in Concurrency:
    • Race Conditions: When multiple processes or threads attempt to modify shared resources simultaneously, leading to inconsistent results.
    • Deadlock: A situation where a set of processes are blocked because each process is holding a resource and waiting for another resource held by another process.
    • Livelock: Similar to deadlock, but the states of the processes involved change continuously, preventing progress.
    • Starvation: A situation where a process is perpetually denied necessary resources.
• Deadlock Prevention:
  • Mutual Exclusion: Ensure that at least one of the resources is non-sharable.
  • Hold and Wait: Prevent a process from holding one resource while waiting for others.
  • No Preemption: Allow the OS to forcibly take resources from a process.
  • Circular Wait: Impose a total ordering of resource types and require that each process requests resources in an increasing order of enumeration.
• Deadlock Detection and Recovery:
  • Detection: Use algorithms to detect cycles in the resource allocation graph.
  • Recovery: Terminate one or more processes involved in the deadlock, or preempt some resources and allocate them to other processes.

4. File Systems

• File System:
  • Definition: The method and data structures that an operating system uses to manage files on a disk or partition.
• File Types:
  • Regular Files: Contain user data (e.g., text, executables).
  • Directories: Contain information about files and other directories.
  • Special Files: Represent physical devices like printers, disks, etc.
• File System Structure:
  • File Allocation Table (FAT): A simple file system architecture that uses a table to keep track of where files are stored.
  • New Technology File System (NTFS): A modern file system used in Windows with advanced features like journaling, file compression, encryption, and support for large files.
  • Inode: A data structure used in Unix-based file systems that stores information about files and directories.
• File Operations:
  • Creating, Deleting, Reading, Writing, and Appending to files.
  • File Attributes: Metadata like file name, size, type, permissions, creation/modification date.
  • Access Control: Permissions and ownership attributes that control who can read, write, or execute a file.

5. CPU Scheduling

• CPU Scheduling:
  • Definition: The process by which the OS decides which process or thread to run on the CPU at any given time.
• Scheduling Criteria:
  • CPU Utilization: Keep the CPU as busy as possible.
  • Throughput: Number of processes completed per time unit.
  • Turnaround Time: Total time taken from submission to completion of a process.
  • Waiting Time: Time a process spends in the ready queue.
  • Response Time: Time from submission to the first response.
• Scheduling Algorithms:
  • First-Come, First-Served (FCFS): Processes are scheduled in the order they arrive. Simple but can lead to the convoy effect.
  • Shortest Job Next (SJN): Chooses the process with the smallest execution time. Can lead to starvation of longer processes.
  • Round Robin (RR): Each process gets a small unit of CPU time in cyclic order. Fair but can have high overhead due to context switching.
  • Priority Scheduling: Processes are scheduled based on priority. Can lead to starvation of low-priority processes.
  • Multilevel Queue Scheduling: Different queues for different process priorities with separate scheduling algorithms.

6. Input/Output Systems

• I/O Devices:
  • Definition: Hardware that allows the computer to interact with the outside world (e.g., keyboards, monitors, printers).
• Device Drivers:
  • Definition: Software that controls a particular type of device attached to the computer.
• I/O Scheduling:
  • First-Come, First-Served (FCFS): I/O requests are processed in the order they arrive.
  • Shortest Seek Time First (SSTF): The request closest to the current head position is served next.
  • SCAN (Elevator Algorithm): The disk arm moves in one direction, servicing requests, then reverses direction.
  • Disk Scheduling: Optimizes the order of disk I/O requests to minimize seek time.

7. Security and Protection

• Security:
  • Definition: Protecting the OS from external threats and ensuring the confidentiality, integrity, and availability of data.
• Authentication: Verifying the identity of a user or device.
• Authorization: Determining if a user or device has permission to perform an operation.
• Encryption: Protecting data by encoding it so that only authorized parties can decode it.
• Threats: Malware, viruses, worms, trojans, phishing attacks, etc.
• Protection:
  • Definition: Mechanisms to control access to resources in the OS.
  • Access Control Lists (ACLs): Lists that define which users or system processes are granted access to objects.
  • Capabilities: Tickets that give a user or process access rights to an object.

8. Networking

• Network Basics:
  • Definition: A network is a collection of interconnected devices that share resources and communicate.
• OSI Model:
  • Layers: Physical, Data Link, Network, Transport, Session, Presentation, Application.
• TCP/IP Model:
  • Layers: Link, Internet, Transport, Application.
• Protocols:
  • TCP: Reliable, connection-oriented communication.
  • UDP: Unreliable, connectionless communication.
• Sockets: An endpoint for sending or receiving data across a computer network.
• DNS, IP Addressing, Subnetting, and Routing: Fundamental concepts for networking.

Study Tips

• Understand Concepts Thoroughly: Focus on understanding the concepts rather than just memorizing them.
• Practice Problem-Solving: Work on problems related to process scheduling, memory management, and concurrency to get comfortable with applying these concepts.
• Use Practical Examples: Try to relate OS concepts to real-world scenarios or projects you’ve worked on.
• Explore System Internals: If possible, explore operating system internals like Linux kernel modules or Windows system internals to see these concepts in action.